Wavelength division multiplexing (WDM) is an efficient way of multiplying the capacity of optical fibre. In wavelength division multiplexing, several independent transmitter-receiver pairs use the same fibre. FIGS. 1a and 1b illustrate the principle of wavelength division multiplexing, using as an example a system having four parallel transmitter-receiver pairs. Each of the four information sources (not shown in the figure) modulates one of four optical transmitters, each of which generates light at a different wavelength (.lambda..sub.1 . . . .lambda..sub.4). As will be seen from FIG. 1a, the modulation bandwidth of each source is smaller than the distance between the wavelengths, and thus the spectra of the modulated signals do not overlap. The signals generated by the transmitters are combined onto the same optical fibre OF in a WDM multiplexer WDM1, which is a fully optical (and often passive) component. At the opposite end of the fibre, a WDM demultiplexer WDM2, which is also a fully optical (and often passive) component, separates the different spectral components of the combined signal from one another. Each of these signals is detected at a discrete receiver. Hence, a narrow wavelength window is assigned for the use of each signal in a given wavelength range. A typical practical example might be a system where the signals are in the 1550 nm wavelength range for example in such a way that the first signal is at the wavelength 1544 nm, the second signal at the wavelength 1548 nm, the third signal at the wavelength 1552 nm and the fourth signal at the wavelength 1556 nm. Nowadays a multiple of 100 GHz (approx. 0.8 nm) is becoming the de facto standard for the distance between wavelengths.
In order to enable a given wavelength channel to be branched off a transmission link using wavelength division multiplexing or a given wavelength channel to be added thereinto, an element called an add/drop filter has been developed. The task of an optical add/drop filter is (1) to direct off a selected narrow-band channel (wavelength) from the optical aggregate signal that passes in the fibre incoming to the filter (drop function) and/or (2) to add to the fibre outgoing from the filter a narrow-band channel (add function). The signals (wavelengths) that have not been selected for dropping pass through the add/drop element from the incoming fibre to the outgoing fibre. Thus a desired narrow-band channel is added or dropped in the filter without otherwise affecting the spectrum of the optical aggregate signal in any way.
FIG. 2 illustrates the structure of a known three-port add/drop filter OADE. References L1 . . . L3 denote port-specific lenses and reference F an interference filter. The incoming fibre is connected to port A, through which a number of wavelength channels (.lambda..sub.1 . . . .lambda..sub.4) arrive. One of the wavelength channels (in this example .lambda..sub.1) passes through the interference filter F (and lenses L1 and L2) to port B. The remaining wavelength channels also pass through lens L1 but are reflected from the interference filter (through lens L3) to port C. The wavelength of the channel entering port B is a fixed, filter-specific constant.
The filter operates in both directions, and hence the adding function is achieved in such a way that the existing channels are fed to port C and a channel to be added to port B, and hence all channels are obtained from port A.
A three-port filter of the kind described above is manufactured for example by Optical Corporation of America, U.S.A.
The present-day optical telecommunications systems based on wavelength division multiplexing are point-to-point transmission systems, but optical transmission technology is being constantly developed to implement the lowest layers of broadband network architectures in the form of fully optical systems by means of which the transmission of high-capacity information streams can be handled fully optically (by means of an optical cross-connect). After point-to-point systems, optical networks suitable for add/drop operations--such as ring networks--constitute the next phase in this evolvement path. To make it possible to flexibly configure such networks in accordance with traffic needs, add/drop filters are replaced by network elements in which the wavelengths to be dropped/added can be selected. Such a network element will be termed an add/drop device in the following. In other words, an add/drop device is a network element that can be so configured that the wavelengths to be dropped/added are selectable.
One optical ring network and the add/drop device used therein is disclosed in European Patent Application 0 651 520. Since the add/drop device in accordance with the present invention is also used in a corresponding optical ring network, such a ring network will be briefly described in the following with reference to FIG. 3. In the exemplary case of FIG. 3, the network is used for the transmission of SDH (Synchronous Digital Hierarchy) signals, but the type of the signal carried by each wavelength may naturally vary. The signal may also be for example a PDH (Plesiochronous Digital Hierarchy) signal or an ATM (Asynchronous Transfer Mode) signal.
The ring network in this exemplary case comprises four nodes consisting of add/drop devices OADM1 . . . OADM4. An optical transmission connection is provided between the nodes, said connection being implemented with optical fibres OF, and the above-described wavelength division multiplexing is used in each one-way link between any two nodes. In this example, four wavelengths (.lambda..sub.1 . . . .lambda..sub.4) and one management wavelength (.lambda..sub.m) are used, but it is to be understood that the number of wavelengths used in the network may vary and can also be much greater. Each add/drop device may have an interface to the control system ONC of the device and/or of the entire optical network, through which the configuration of all add/drop devices in the network can be set.
The management system is otherwise located in the SDH equipment, but the configuration of the add/drop multiplexers can be handled through the optical network management system. The optical signal arriving from an SDH device is connected to the ring network at the desired wavelength. In accordance with traffic needs, the desired number of wavelengths is set between the nodes. In other words, the routing configuration of the signals corresponding to the different wavelengths can be altered in accordance with the traffic situation. By means of a management signal travelling at the management wavelength, the nodes are configured in such a way that the desired wavelengths are dropped/added in each node. As is shown in the figure for node OADM1, a drop element MCD can be dedicated for the management wavelength of the node in each transmission direction, said element dropping the management channel operating at the management wavelength Am into the node control unit CU, which again converts the signal into electrical form and controls the add/drop part AD of the node as indicated by the management signal in such a way that the desired channels are dropped into the SDH equipment and the desired channels are added from the SDH equipment. In addition to the drop element, each transmission direction has an add element MCA that adds the management channel arriving from the control unit CU to the outgoing signal leaving the node.
Node management may also take place directly via a local management interface (optical or electrical) in such a way that the management of each node is performed separately through a local interface, or both a local management interface and a network management channel can be used for node management in such a way that a signal can be connected via the local management interface onto the network management channel.
At every wavelength, for example an STM-N (Synchronous Transport Module) signal, such as an STM-16 signal (N=16), can be transported. In the example of FIG. 3, it has been simply presumed that wavelength .lambda..sub.1 corresponds to SDH device SDH1, wavelength .lambda..sub.2 corresponds to SDH device SDH2, wavelength .lambda..sub.3 corresponds to SDH device SDH3 and wavelength .lambda..sub.4 corresponds to SDH device SDH4.
The ring network may further comprise a protection system implemented with optical switches, so that when there is a failure on a link between two nodes, a transmission connection can be established through the remaining ring.
In the European patent application referred to above, in the add/drop device serving as a node in the ring network, incoming fibre is connected to an optical demultiplexer that separates each incoming wavelength onto a discrete optical conductor. The output side of the device has an optical multiplexer, and several optical conductors each transferring a specific wavelength are connected as inputs thereof. The multiplexer combines the incoming wavelength channels onto the outgoing fibre.
One drawback of such an add/drop device is that it does not allow flexible implementation of changes in the system, such as addition of wavelength channels. This is due to the fact that the wavelength channels are separated in the same demultiplexer component and are combined in the same multiplexer component, in which the number of wavelengths and the wave length values are preset parameters. Hence, the multiplexers and demultiplexers of the add/drop devices must be changed for example when it is desired to add one channel to the system.